A rod drawn positive displacement pump (otherwise known as a downhole sucker rod pump) in use is located at the bottom of a string of tubing in a wellbore. The plunger of the prior art pump is reciprocated by a string of sucker rods extending down through the tubing. A pump jack at ground surfaces reciprocates the sucker rod string.
The basic parts of the prior art are shown schematically in prior art FIG. 1. They comprise: a cylindrical tubular outer barrel 20 communicating at its upper end with the tubing bore; a cylindrical tubular plunger 24 working within the barrel 20 and communicating at its upper end through ports 26 with the bore 28 of the barrel; and standing and travelling valves. The standing valve 30 is positioned usually in the bottom end of the barrel 20 and controls the entry of fluid into its bore 28. The travelling valve 32 is positioned usually in the bottom end of the hollow plunger 24 and controls entry of fluid into its bore 28 on the up stroke of the plunger 24 (FIG. 1a), fluid moves from the reservoir through the open standing valve 30 and into a "compression chamber 34", which is that portion of the barrel 20 bore between the two valves 30 and 32. As the plunger 24 reverses its travel and begins to move downwardly on the down stroke (FIG. 1 b), the standing valve 30 closes and the fluid in the compression chamber 34 is compressed. When the pressure in the compression chamber 34 is greater than the pressure exerted on the ball 32 of the travelling valve by the column of fluid standing above it in the tubing bore 28, the travelling valve 32 opens. The fluid in the compression chamber then moves through the travelling valve as the piston 24 falls through it (FIG. 1c). When the piston 24 reverses and starts back up (FIG. 1 d), the travelling valve closes 32 and the fluid above it is lifted toward ground surface. Also, a new cycle of pumping is initiated because the standing valve again opens to admit fluid from the reservoir into the compression chamber 34.
Although only a relatively small amount of fluid is pumped with each stroke, the production of a well can be large as it is worked 24 hours a day at a constant rate. It is common for a pump to be stroked 10,000 times a day.
The pump valves 32 and 34 experience widely varying conditions depending on factors such as stroke length and speed, oil viscosity, gas to oil ratio and particulate matter content in the produced fluid. The conditions arising from these factors and affecting the valve performance include fluid velocity, pressure drop, gas locking, fillage time, sand erosion, stroke loss and the like. One of the objectives of the present invention is to modify the valve cage to provide a valve that is better adapted to cope with these factors and conditions to yield a better performing valve and pump.
At this point is useful to shortly describe the parts of a conventional standing or travelling valve. These valves basically are one form of a ball check valve. More particularly, having reference to prior art FIG. 2a and 2b, the valve comprises a cage. The cage is generally tubular in configuration and defines an axial bore 35 extending therethrough. Adjacent its lower end, the cage wall forms an inwardly protruding, internal, annular seat shoulder 36. A valve ball 38 and seat 40 are inserted through the bottom of the bore 35. The seat 40 abuts the seat shoulder 58. A lock ring or seat retainer 42 is threaded into the bottom of the bore 35, to lock the seat ring 40 in place. The valve ball 38 is thus positioned above the secured seat 40 and can seal against an annular lapped sealing surface on the upper side of the seat orifice. A stop 44 extends transversely across the cage bore 35 at a point spaced above the seat 40. Commonly the stop 44 is a horizontal bar extending between opposed points on the cage side wall 46. The stop 44 is integral with the side wall 46. It functions to limit upward travel of the ball 32. A plurality of internal, elongated, circumferentially spaced apart, inwardly protruding guide ribs 48 extend between the seat shoulder 36 and the stop 44. These guide ribs 48 form a race of constant diameter, for closely guiding the ball 38 so that it moves only vertically. The side wall 46 thickens inwardly where the stop 44 is formed and this reduced bore diameter section 50 continues on up to the top end of the cage, with few exceptions. An external thread 52 is formed in the top end of the outer surface of section, for connection with the barrel or plunger.
The conventional valve shown in FIG. 2 has been in existence and widely used for a number of decades.
As a result of working with these valves for many years, the present applicant has identified a number of characteristics that would be desirable to incorporate into them. More particularly, it would be desirable:
(a) To bring the standing and travelling valve seats closer together at the bottom of the down stroke, to improve compression ratio and reduce gas locking; PA1 (b) To provide increased flow area in the valve, to reduce pressure drop and thereby improve fillage; PA1 (c) To automatically vary the flow capacity of the valve in response to variation in the flow rate of the fluid seeking to move through the valve, to diminish pressure drop and reduce stroke loss; PA1 (d) To reduce spalling of the ball, which can occur when the ball contacts the conventional stop bar; and PA1 (e) To induce the fluid to spin as it passes through the cage, to thereby increase the "hang time" of entrained solid particles in the liquid and keep them suspended, rather than having them settle onto the valve parts. PA1 The flow area around the valve ball automatically increases as the volume of fluid seeking to move through the valve increases. This helps to reduce increases in pressure drop across the valve; and The ball is kept closer to the seat than would be the case if the flow area did not increase. By keeping the ball close to the seat, stroke loss can be reduced (stroke loss refers to fluid lost back to the reservoir during the momentary interval when the ball is dropping and sealing at the beginning of the down stroke). PA1 The valve seats of the travelling and standing valves are now typically spaced apart about 4 inches at the bottom of the down stroke. (By way of comparison, the prior art units have a seat separation in the order of 5-8 inches at the bottom of the down stroke.) By incorporating this modification into a pump, the compression ratio is much increased and the likelihood of failing to create sufficient pressure to unseat the travelling valve ball is reduced; PA1 Since the stop means have been moved to the top of the cage, it has been necessary to move the external threads downwardly, to a segment of the side wall where the cage side wall is relatively thick.
The present invention is directed toward modifying the valve cage so that the valve and pump incorporating it can acquire some or all of these characteristics and their benefits.